gh-154786: Fix crashes on a screen without a terminal (GH-154787)

gh-154786: Fix crashes on a screen without a terminal (GH-154787)

screen.use() makes its screen current for the callback, so a new_prescr()
screen, which has no terminal, can be current without set_term().
Operations that need a terminal then crashed inside curses.

Raise curses.error instead, checking that stdscr exists.